Frost & Frost Pockets

Having some understanding of frost might make the difference between you saving a precious plant or not.

This article has been written to try and pass on this understanding in a way that will allow you to take preventative measures to save / protect your plant/s.

What is frost?

As air cools at night, the soils ability to hold water vapour decreases, and the heat accumulated in the soil radiates back into the atmosphere.

If the air continues to cool, excess vapour condenses, and forms as dew on the surface of plant leaves.

This dew then freezes if the temperature continues to drop to 0°C (32°F) or lower.

This loss of heat continues until just after dawn, i.e. until such times as the sun warms it up again, so this is why the sharpest frosts often occur in early morning.

Plants can be severely damaged by extended frost periods due to the water inside the plant cells freezing and rupturing the cell walls.

Many hardy plants react to these periods, by lowering the freezing point of their cell contents, whereas other plants simply drain much of the moisture from their cells during winter months, and rehydrate them in spring.

Frosts are often referred to as;

Light frost: this can be as little as minus 1°C (30°F)

Severe frost: is considered to be minus 12°C (10°F) or lower.

Air frosts: is when the ground retains heat, but the air above ground level freezes.

Ground frosts: occur when the ground level freezes.

Hoar Frost: is formed when the air is humid.

Frosts are generally associated with periods of high pressure where during the day the skies are sunny and / or clear then followed by cold night.

What is a frost pocket?

It is cold air trapped close to the ground.

Cold air is denser than warm air and tends to flow downhill, i.e. into depressions or against fences, walls and hedges.

It is in such conditions that cold air becomes trapped by the warmer air above it.

Plant Hardyness:

In 2011 The Royal Horticultural Society (RHS) produced a table (see here) to rate the degree of hardyness that one might see on a seed packet, plant label or in the gardening press indicating the hardyness of a plant.

*Each rating indicates the range of temperature plants need to be kept in.

H1 - Tropical:

Best grown in a heated greenhouse or as a house plant that is warmer than 15°C (> 59°F)

H1 b - Subtropical:

Should be grown in a heated greenhouse or indoors where a mimium temperature of between 10°C to 15°C (50°-59°F) can be maintained.

H1 c - Warm temperate:

Should be grown in a heated greenhouse, conservatory or as a house plant where a mimium temperature of between 5°C to 10°C (41°-50°F) can be maintained.

H2 - Tender:

Can be kept in a cool or frost-free greenhouse in winter at a minimum temperature of between 1°C to 5°C (34°-41°F)

H3 - Half-hardy:

Can be kept in an unheated greenhouse or outdoors in mild areas at a mimimum temperature of between minus 5°C-1°C (23°-34°F)

H4 - Hardy:

Can be kept outdoors during an average winter where the temperature gets to between minus 10°C to minus 5°C (14°-23°F)

H5- Hardy:

Can be kept outdoors in a cold winter where the temperature gets to between minus 15°C to minus 10°C (5°-14°F)

H6- Hardy:

Can be kept outdoors in a very cold winter where the temperature gets to between minus 20°C to minus 15°C (minus 4° to +5°F)

H7 -Very hardy:

Will survive outdoors during the worst UK winters e.g where temperatures get lower than minus 20°C or minus 4°F.

*This table should only be used as a rough guide simply because: in some situations, choose where you live in the UK, you can encounter extremes in local weather conditions.

In my garden which is roughly 15m wide and 40m from front to back, temperature ratings can vary from one area of it to another, meaning I have always got to be aware of this when planting out plants with different hardyness ratings.

For example: The back garden faces South, yet the East side of the garden is in a Frost Pocket caused by a boundary fence.

The west side is in relative shade, and is generally much warmer because of the 2m high hedge that runs along the boundary.

The front garden faces North and is shaded by the house so conditions can be somewhat different here also.

The East side in Mid morning

My advice would be: “Get to know your own garden” in terms of frost exposure, and grow your plants accordingly!

Another geographical factor could be that in Coastal districts as opposed to an Inland area, you might experience milder winter temperatures brought on by the warming effect of the sea.

Or you may be facing East and get the full brunt of the cold winds that cross the North Sea from Siberia, conversely you may face West and get some benefit from the Gulf Stream.

Altitude and location can be another factor as some gardens can be at or below sea level, or around 250m (800ft) above sea level as my garden is.

UK Weather Map

There is no simple answer to this phenomena other than as mentioned previously: Get to know you garden!

In general terms the first frosts in the UK are typically encountered anytime from September to November, depending on your geographical location.

Similarly the last frosts can be as early as April in milder districts, or as late as June elsewhere.

Frost Measurement:

Meteorologists normally record air temperatures at an average height of 1.5m (5ft) above ground level, however! the ground temperature below the thermometer can be as much as 3-5°C lower!

Example; If reading at thermometer level is say 3°C (37°F) then actual* ground level temperature could be around -1°C (30°F)

*This variance might be much less if the night is cloudy, and or if there is any wind present.

The type of the soil can also have an influence on the severity of a frost.

For example: Sandy soils lose heat to the air more rapidly than a Clay soil.

Similarly all soils are constantly losing heat so this can reduce the severity of the frost at ground level.

Having said that: after periods of continuous frost these variations decrease.

DIY forecasting:

Apart from listening to the weather forecast on the TV or Radio or checking on line, you can often get a good indication of when frost is likely by doing some measurements yourself.

To do this mount an outside thermometer in the shade, preferably facing northeast, and set it at 1.5m (5ft) above ground level.

A post driven into a flower bed will make a good spot to hang the thermometer.

Do not fix it to a heated building, as heat radiating from the building will affect your readings.

Take temperature readings at say 2pm (x) and 7pm (y)

To calculate the expected Dawn Temperature (DT) use the equation DT=2y-x

That is; double the temperature at 7pm (y) then subtract the 2pm reading.(x)

Example; when X = 14°C (57°F) and Y= 8°C (46°F)

In Celsius the DT = 2 x 8° minus 14° or (16-14) = 2°C

In Fahrenheit the DT =2 x 46 minus 57° or ( 92-57) = 35°F

In this example: one could expect a ground frost!

These are Air temperatures and as we know that temperatures can be 3°-5°C .(5°-10°F) degrees lower at ground level i.e. circa -1°C (27°F)

Do’s & Don’ts:

The following procedures may help to reduce the effect of frost damage:

Eliminating frost pockets.

If the condition is due to a slight hollow or slope, improve the situation by filling in the hollow or levelling out the slope, this will allow the cold air can flow over / through the garden.

To reduce the effect of solid barriers such as walls, fences or solid hedges, create an opening at the lowest point to allow the air to escape.

Impede the passage of cold air flowing into the garden from higher ground by erecting a slatted fence, hedge or line of small trees / shrubs at the top of the incline.

Protection /Prevention:

Find out the average dates of the first and last frosts in your area. (This chart may help!)

Have insulating materials at hand to cover plants!

Listen carefully to weather forecasts.

If a frost is forecast cover susceptible low-growing plants with some form of insulation such as agri-fleece, this can keep the temperature at plant level a few degrees warmer.

Some plants are particularly vulnerable to late-spring frosts if positioned where they receive the early-morning sun.

This type of damage is the result of the rapid warming of the plant which causes damage to the plant's leaf tissue.

In the event that a plant/s gets frosted in this manner, spray it / them with cold clean water before the sun rays reaches them, this will lessen the risk of sun damage.

Try not to cause plants any physical stress when frosted.

For example;

Avoid walking on frozen grass as this may crush the cells in the blades of grass.

Delay cutting back perennials and grasses until spring, collapsed plant material will protect new shoots.

The root balls of plants in pots are susceptible to freezing, wrap the pots in bubble wrap prior to the onset of winter or shelter them in a garage or porch.

Remember to water potted evergreens during milder spells, freezing weather can restrict uptake of water, which is still required in winter.

Cover small fruit trees and bushes with fleece overnight to protect buds and flowers from late spring frosts; remove during the day to give pollinators access.

Cloches can also be used to protect low growing plants such as strawberry plants.

Moist soil can hold as much as four times the heat as dry ground e.g. air temperatures just above recently soaked soil are as much as 3°C (5°F) higher than over dry ground.

Meaning, if frost is forecast, watering a seed bed early in the day then covering with fleece or cloches will help protect young / tender plants.

When planning a garden in colder areas, try and select tallish hardy plants that start into growth or flower late, these tend to hold their flowering and fruiting shoots above the cold air near soil level.

Delay pruning until cold weather is past this will avoid the moist open pruning wounds being affected by frost.

More tender plants thrive higher up the slope, where air is warmer, and drainage is often better.

Similarly; Planting against a warm south wall will also raise the temperature around the plant/s.